DVB Revising the DVB-S and DVB-DSNG Digital Satellite Transmission Standards
May 12, 2003
The DVB-S and DVB-DSNG digital satellite transmission standards are the basis of many digital satellite transmission networks, whether they are used for contribution, distribution or broadcasting. Users are demanding more bandwidth and a key goal of the new set of standards, being called DVB-S2, is to "enable delivery of a significantly higher data rate (e.g. 30%) in a given transponder bandwidth than the current DVB-S standard."
Recent reports indicate DVB-S2 will be based on 8PSK modulation using low density parity check (LDPC) coding instead of more commonly used "turbo-coding" error correction. Coding is critical because 8PSK (8 phase shift keying) packs 3 bits of data in each symbol, compared with 2 bits for QPSK (quadrature phase shift keying). The additional modulation states make the signal more susceptible to noise and interference. Although 8PSK is 50% more bandwidth efficient than QPSK, after the more robust coding is taken into account, the efficiency improvement drops to the 30% range.
According to the DVB TM-S2 web site http://www.dvb.org/index.php?id=75, three application areas are being considered in developing DVB-S2:
* Video and audio broadcasting (including a backwards compatible mode allowing "old" DVB-S receivers to decode at least part of the transmitted multiplex)
* Interactive services (direct path), such as Internet access, for the consumer market
* Professional links (SNG, contribution, Internet Trunking)
The broadcast profile specification for DVB-S2 is expected to be defined next month, with the full specification completed by the end of the year.
If your familiarity with digital coding methods ends with the ATSC standard, visit the Coding Research Group at the University of Notre Dame web site at http://www.nd.edu/~eecoding/pub1.html for links to publications and web sites with information on turbo codes. An excellent, easy to read, easy to follow, presentation on LDPC coding that includes a comparison with other coding methods is available at http://grouper.ieee.org/groups/802/20/Contribs/C802.20-03-02R1.pdf.